Abstract: The MELCOR1.8.6 code was applied to a severe accident model of a 1 000 MWe PWR which includes primary system, secondary system, passive core cooling system and containment system. For the transient case, a small break LOCA with 2 inch (5.08 cm) break at the cold leg concurrent with failure of gravity injection was selected. After the core was damaged due to the failure of gravity injection, it was assumed that the coolant was injected into the pressure vessel, and then the water reflooding effectiveness was evaluated and analyzed. In this calculation, the coolant injection into reactor core with the small (10 kg/s), medium (50 kg/s) and large (200 kg/s) mass flow rates respectively at 3 different time stages of the severe accident was simulated. The effectiveness of water reflooding was assessed through hydrogen production, radioactive materials released from core, and core temperature. The results show that the mass flow rate above 10 kg/s is believed to be efficient for cooling a 1 000 MWe reactor at the beginning of core damage. However, with the accident developing to core relocation, a large mass flow rate of 200 kg/s is considered to be applicable for core cooling. As a result, the mass flow rate below this value should be carefully considered when injecting water into the core.